(1) Field of the Invention
The present invention relates to the purification of aqueous solutions of sodium hydroxide such as are obtained as products of the electrolysis of sodium chloride.
(2) Description of the Prior Art
Alkali metal hydroxides such as sodium hydroxide are generally produced by the electrolysis of salt brines, such as sodium chloride solutions in electrolytic cells. These solutions also contain a substantial amount of alkali metal chloride and alkali metal sulphate, for instance sodium chloride and sodium sulphate, as contaminating impurities. A typical cell effluent from the electrolysis of sodium chloride will contain about 8 to about 12 percent by weight of sodium hydroxide, about 17 percent sodium chloride, and about 0.2 to about 1.5 percent by weight sodium sulphate with the balance as water. At higher concentrations of sodium sulphate some reduction occurs in the particle size of the sodium chloride, as is well known. For instance, at a concentration of 4 to 7% sodium sulphate, the sodium chloride is obtained at a particle size of about 200 to about 270 microns, while at a concentration of 2% sodium sulphate, the sodium chloride is obtained at a particle size of about 300 to about 330 microns. The sodium sulphate is not affected by the electrolytic action which takes place in the electrolytic cell and remain in solution and continues to build up where the sodium hydroxide solution is recycled through the electrolytic cell. The increasing concentration of sodium sulphate can have a deteriorating effect on the graphite electrodes in the cell.
Procedures for removing sodium sulphate from sodium hydroxide solutions are known. The sodium hydroxide solution cell effluent can be directed to a series of evaporator tanks where the solution is concentrated by steam evaporation such as to a concentration of about 20 to about 35 percent by weight of sodium hydroxide at which concentration both sodium chloride and sodium sulphate contained in the solution will crystallize out of the solution. The resulting slurry is then directed to a flat bed filter where the mother liquor is extracted under vacuum and the remaining salt mass is washed and purged with water to separate the sodium chloride from the sodium sulphate so that the sodium chloride solution can be recycled through the cell system.
In another method of separating sodium sulphate from a sodium hydroxide aqueous solution, a hot caustic electrolytic cell effluent solution having a concentration of about 20 to 35 percent by weight of sodium hydroxide is directed from an evaporator to a heat exchanger where it is cooled to a temperature of about 0.degree. C. At this temperature and concentration, a portion of the sodium sulphate will precipitate out of the solution as sodium sulphate decahydrate. The solution remaining after the first precipitation is recycled back to the evaporator for further concentration and return to the heat exchanger where it is again cooled to precipitate additional sodium sulphate.
Both of these procedures involve a series of time-consuming steps and a substantial amount of costly equipment and therefore to make the production of purified sodium hydroxide more commercially feasible a genuine need exists for a simpler and more efficient method for removing the sodium sulphate impurity from sodium hydroxide aqueous solutions. The solution disclosed in U.S. Pat. No. 3,423,187 is the precipitation of the sulphate ion from the aqueous sodium hydroxide solution following the addition of anhydrous ammonia in either a gaseous or liquid form. In this process the ammonia is dissolved in a sodium hydroxide aqueous solution having a concentration of up to about 50 percent by weight. The pressure factor is of particular importance to this method of purifying the sodium hydroxide solution since the more concentrated sodium hydroxide solutions are known to exhibit a relatively low solubility for ammonia at atmospheric pressure. It will therefore be appreciated that such a system of purification, which generally involves the use of pressurized vessels, is still complex and a simpler and more efficient method for removing sulphate ion impurities in aqueous sodium hydroxide solutions is needed.
The formation of double salts of sodium carbonate and sodium sulphate is disclosed in Chemical Abstracts, Vol. 44, page 920 and Vol. 48, pages 3131 and 9172. The double salt is identified as burkeite (2Na.sub.2 SO.sub.4.Na.sub.2 CO.sub.3) on page 3131. There is no indication that the particle size of burkeite is such that it can be more easily removed from aqueous slurries as compared to particles of sodium sulphate.
U.S. Pat. No. 4,277,447 is of interest in disclosing a method for reducing calcium ion concentrations in alkali metal chloride brines by the formation of an insoluble calcium salt upon the addition of an alkali metal carbonate.